Replace an owned agent's spending policy wholesale

Chain-root only (or an agent.spending delegate acting for it). Last-write-wins (no CAS); every write lands in the event stream. A delegate may lower caps, remove rules and freeze freely; RAISING anything is refused 403 (cap_exceeded) above the mandate's max_cap (a per-MONTH ceiling in one currency; a rule counts as raised unless it is provably no looser than the same chain's current rule) and a delegate can never lift a freeze here (use POST /agents/{id}/resume).